Miniature and sub-miniature connectors, as utilized at the present time, generally comprise a threaded outer conductor shell which is normally made from stainless steel and a center conductor of the spring finger collet type, normally manufactured from a suitable highly conductive material such as beryllium copper, and a dielectric bead support for the center conductor which extends the length of the spring finger center conductor collet. An example of such a structure is shown in U.S. Pat. No. 3,147,057 of Collusi, issued Sept. 1, 1964.
At least two problems arise with regard to the structural operation of connectors of the type shown in the above referred to patent. First of all, the outer conductor shell of the connector, which is made from stainless steel, has lossy characteristics at the RF signal frequencies for which such connectors are often used and, consequently, the signal insertion losses thereof are undesirably high, particularly at higher frequencies. In order to counteract such insertion losses at higher frequencies, the conductive shell, manufactured from stainless steel, is plated with a precious metal such as gold, or silver, or a combination thereof in order to increase the conductivity thereof and to negate the lossy RF characteristics. Plating of the stainless steel with precious metals to improve the loss properties at higher frequencies has a tendency to flake off at the interface as a result of the inter-connect with the mating connector. The fine particles of precious metal which flake off become deposited on the dielectric interface causing a deterioration in RF performance. Because of the mechanical problems and expense associated with such plating of the outer conductive shell with a soft precious metal, manufacturers often omit the plating material so that the unplated shell, having increased transmission line losses especially at higher frequencies, becomes less effective in applications requiring low insertion loss over a wide range of frequencies.
Secondly, in the construction shown in the Collusi patent, the dielectric bead therein tends to move within the outer conductor shell, the position of the center conductor collet thereby moving axially in connector. Such a shift in position of the center conductor and the dielectric bead results in improper mating and orientation of coaxial cable components and produces undesirable electrical discontinuities which result in unwanted reflections of the transmitted signal.
In order to avoid the axial movement of the dielectric bead and, hence, of the center conductor, manufacturers have utilized a structure in which a plastic pin extends from the outer conductor shell through the dielectric bead to a region where it mechanically engages with the center conductor collet. Such structures, for example, are shown in U.S. Pat. No. 3,292,117 of Bryant et al., issued on Dec. 13, 1966. Thus, captivation of the spring finger center conductor collect occurs either through, or around, an annular groove which is cut into the center conductor member as shown therein.
Because of the shear strength and other mechanical properties which are required in order to enhance the life of the threads on the outer conductor shell, which shell has a relatively thin cross-section, stainless steel is normally used for such outer conductive shell. Accordingly, the stainless steel shell is normally plated with a precious metal in order to increase the conductivity of the inner diameter transmission line path thereof and to reduce the transmission line losses. As discussed above, such precious metal plating tends to be removed, or to flake off, as interconnections are continuously made and un-made between the connector and the coaxial cable with which it is mated. The precious metal also tends to become deposited on the dielectric interface between the coaxial cable and the connector thereby causing further deterioration in the performance of the connector at the frequencies normally of interest in miniature and sub-miniature connector applications. If the precious metal plating is omitted, unsatisfactory insertion loss characteristics arise, which losses, as discussed above, tend to increase as the frequencies of the transmitted signals increase.
Further, operation of such connectors at high temperatures causes an expansion of the dielectric support member so that it tends to extend beyond the shoulder formed at the end of the threaded outer conductor shell and causes improper connector mating interface operation, again resulting in poor performance at the desired frequencies of operation. The presence of the plastic pin which, for example, may be made of an epoxy plastic causes a discontinuity to occur within the frequency range of interest due to the undercutting of the spring finger center conductor collet where it engages therewith. Further, differences in the dielectric constant of the epoxy pin material and the dielectric bead support material (usually made of Teflon) cause further RF discontinuities to be present. Moreover, it has been found that the threaded conductor shell opening, the center conductor member undercut portion and the epoxy pin material all tend to resonate at random frequencies so as to cause further RF signal discontinuities and distortions. Because of the presence of the epoxy pin, the strength of the outer conductor shell and the center conductor is decreased, particularly when used in miniature and sub-miniature connectors in which the diameters are relatively small. Since the shear strength of the epoxy material is much less than that of stainless steel, the strength of the overall structure is accordingly reduced.
Efforts to avoid the use of an epoxy pin structure or equivalent have been suggested as in U.S. Pat. No. 3,372,364 of O'Keefe et al., issued on Mar. 5, 1968. In the O'Keefe structure a pair of ring members are wedge fitted within the outer conductor shell on either side of a dielectric insert member which holds the center conductor member so that the dielectric insert is effectively captured therebetween and prevented from moving axially within the outer conductor shell. The dielectric insert extends from the center conductor member to the outer conductor shell. The use of such ring members causes a discontinuity at the interface thereof with the dielectric insert and, hence, gives rise to undesired reflections of the transmitted signals thereat. Further, the use of that portion of the outer shell in contact with the dielectric insert as the outer conductor of the connector produces transmission losses particularly at the high frequencies of operation that are desired in miniature and sub-miniature connectors, which losses can only be counteracted by plating the interior surface of the outer conductor shell with a precious metal as discussed above.
Moreover, when coaxial cables are mated with the connector of O'Keefe et al., pressure differences can occur across the dielectric insert which tend to distort the shape thereof and cause further changes in the signal transmission properties, which changes can cause reflections in the transmitted signal particularly at high frequencies.
In summary, both the Bryant design and O'Keefe design have inherent characteristics which are detrimental to high frequency operation. In the Bryant design, because the dielectric bead support runs the entire length of the center conductor spring collet, the cross sectional dimensions which are chosen consistent with the dielectric constant of the bead support and consistent with the inter-connect dimensional requirements, the maximum theoretical operating frequency is limited to approximately 35 GHz.
The O'Keefe design has several inherent characteristics which tend to be undesirable for high frequency operation and preclude interface with present "state of the art" plug connectors. Thus, plating of the outer shell is required and, because the inner diameter of the outer shell is used as the transmission line path, it is not feasible to select cross sectional interface diameters which can adapt to state of the art sub-miniature connectors.